In communication systems, noise and other phenomena of the transmission medium can cause variations in the power of the received signal. Automatic gain control (AGC) circuits respond to this received signal to generate a gain adjusted output signal having nominally constant average power. During operation, the AGC constrains the amplitude of the gain adjusted output signal to a predetermined amplitude range. Many communication systems rely on the utilization of the entire amplitude range to achieve efficient performance. Receiver systems incorporate an AGC to provide the capability for fully utilizing the amplitude range. Illustratively, an AGC attenuates a received signal whose amplitude is outside of the range, thereby, constraining the received signal to the amplitude range. As the received signal decreases in power the AGC decreases the attenuation applied to the received signal proportionally to allow for a fuller utilization of the amplitudes range.
Many circuits are known which automatically adjust the amplitude of a received signal in order to generate a gain adjusted output signal having nominally constant average power. For the most part, these circuits are completely analog and increase in complexity as the need for precision increases. Increased complexity is also directly reflected in increased costs. Digital implementation of the automatic gain control circuit utilizing medium-scale integrated logic is a desirable approach for overcoming the above-mentioned limitation of the prior art.
One prior known AGC arrangement which is implemented by employing digital circuit techniques is disclosed in U.S. Pat. No. 3,981,005 issued to J. Takayama et al. on Sept. 14, 1976. Takayama et al. employ a digital up/down counter and associated threshold control logic elements to control the attenuation applied to the received signal. Specifically, when the amplitude of the received signal is above a predetermined maximum amplitude, the counter is incremented to cause a step increase in the attenuation applied to the received signal. The counter is decremented to cause a step decrease in the attenuation applied to the received signal when the amplitude of the received signal is below a predetermined minimum amplitude. The maximum amount of attenuation which can be applied to a received signal depends on the step size of each attenuation adjustment and the total number of attenuation steps. In the Takayama et al. arrangement, the total number of attenuation steps is related to the number of stages in the up/down counter. A fundamental limitation of the Takayama et al. arrangement is that the attenuator uses large steps of attenuation in order to provide a wide dynamic attenuation range. Reducing the step size of each attenuation adjustment does not overcome this limitation since it reduces the dynamic attenuation range, i.e., the total amount of attenuation which can be applied to the received signal.
One digital AGC which overcomes some of the limitations of the prior Takayama et al. arrangement is disclosed in U.S. Pat. No. 4,052,598 issued to R. J. Turner et al. on Oct. 4, 1977. In the Turner et al. arrangement, the attenuation function is divided between two attenuators controlled by the same control signal. By dividing the attenuator in this manner, Turner et al. provide the capability for rapidly changing the amplitude of the gain adjusted signal.
A problem common to these prior known digital AGC arrangements is the perturbation of the gain adjusted signal caused by one or several attenuation adjustments. Attenuation adjustments are made in response to variations or oscillations of the received signal. When a received signal level exceeds a predetermined upper limit or threshold, for example, the attenuation is increased by a fixed amount or step. Such a step change in attenuation causes small signal perturbations or "glitches" to occur on the gain adjusted signal. Glitches are a source of errors for a system of which the digital AGC is a part. As the received signal level exceeds the upper threshold or drops below a lower threshold more frequently, the occurrence of glitches in the gain adjusted signal increases proportionally. In turn, this increases the probability of error in further processing of the gain adjusted signal in, for example, a data system.